The disease downy mildew on impatiens is caused by Plasmopara obducens. Like Pythium and Phytophthora, Plasmopara obducens is an oomycete, or water mold. The spores have a tail which allows them to swim through water. Spread of infection can progress quickly in wet, moist conditions.
Impatiens downy mildew (Plasmopara obducens) has been confirmed on Impatiens walleriana from landscapes in Oregon. There are unconfirmed reports of the pathogen in landscapes in the Portland Metro, Salem, and Corvallis areas. The Oregon Department of Agriculture is monitoring nurseries associated with the sales of infected impatiens.
There were reports of downy mildew on impatiens in landscapes in several regions of the country in 2011, including several states in the northeastern US, midwest, and southern coastal California. Greenhouse growers and landscapers in cool, wet areas of the country, are encouraged to increase their knowledge and monitoring for this disease.
All cultivated varieties and hybrid species of Impatiens walleriana are susceptible to the disease. New Guinea impatiens, Impatiens hawkerii, are highly tolerant to the disease. Symptoms of the disease include a subtle stippling and yellowing or chlorosis of the leaves. Leaves tend to curl downward. Plants may lack vigor or are stunted. A white coating of spores, generally occurs on the leaf underside but buds and stems may be coated as well. The plants eventually defoliate.
Impatiens downy mildew moves primarily by dispersal of spores. The disease is not seed-born. Like other oomycetes such as Pythium and Phytophthora, Plasmopara has zoospores which can swim in water and thrive in moisture. These spores, produced in sporangia on the undersides of impatiens leaves, can become airborne or carried with free water to infect new plants at an alarming rate. In addition, longer-lasting resistant spores, oospores, may survive in the soil.
The key to managing this disease is prevention. Check out the resources below. American Floral Endowment is taking a very visible lead as a clearinghouse of information on this emerging disease issue.
Anon. 2012. Controlling Downy Mildew on Impatiens walleriana. American Floral Endowment.1 August 2014.
Creswell, Tom. "Stealthy Disease of Impatiens May Go Unnoticed" Purdue Plant & Pest Diagnostic Laboratory. 26 Sept. 2011. Web. 9 Feb. 2012.
Hausbeck, Mary. How to manage downy mildew in the landscape.
Hausbeck, Mary. "Downy mildew causing problems on impatiens in the landscape." Michigan State University Extension News. 12 Oct. 2011. Web. 12 Oct, 2011
Pscheidt, J. 2012. Impatiens Downy Mildew. PNW Plant Disease Handbook. 17 October 2012.
Warfield, Colleen. "Downy mildew of impatiens." GrowerTalks. Vol. 75. no. 10. 9 Feb. 2012. Web. 9 Feb. 2012.
Page last modified 11/7/14